There have been developed furnaces as a reflow furnace, a firing furnace and a drying furnace, which isolate invasion of an external atmosphere and perfectly control the atmosphere in the furnace so as to prevent influences by oxidation under operations therein.
In view of conventional furances that, although a good deal of atmospheric gas is supplied, the oxygen concentration is still high (if N.sub.2 gas is supplied into the furnace at the flowing amount of 300 to 500 Nl/min, the oxygen concentration could not be lowered to 500 ppm; herein Nl: the amount at 0.degree. C. and 1 atmospheric pressure under standards conditions), the inventors proposed a method and a furnace structure where the oxygen concentration could be considerably decreased with a less amount of using an atmospheric gas.
A proposed technological content is that the atmosphere is controlled and the internal pressure therein is made high for checking invasion of the atmosphere, but since the isolation is still insufficient therewith, a complete isolation is made between the zones and between the inside and the outside of the furnace, and only when a work to be treated passes, the isolation is temporarily broken, thereby to check the atmospheric invasion nearly zero, and by sending the work intermittently, any undesirable influence is not given to the isolation.
As a result, the oxygen concentration could be lowered to an extent which has been unexpected in the prior art. (In the example, when N.sub.2 gas was supplied into the furnace at the flowing rate of 80 Nl/min, the oxygen concentration of 15 ppm could be realized.)
A reflow furnace is used to solder to mount parts (mainly LSI) together on a surface of a printed wiring plate. In a case of applying the above stated furnace structure to the reflow furnace, when performing the reflowings on both upper and under surfaces of the plates in response to mounting the parts on the both surfaces, which has recently often carried out, intermittently sending device for practising the reflow has following conditions. Since both surfaces of the work must be exposed to the atmosphere within the furnace, the intermittently sending device must have contacting areas as small as possible at the upper and under surfaces.
In the above mentioned structure, a walking beam system has now been employed as the intemittently sending device. The walking beam is, as shown in FIG. 5(a), composed of stationary beams 80 laid in parallel and movable beams 81 which moves in rectangle. A work X is moved on the movable beams 81 while the beams 81 rise and move horizontally, and on the other hand, while the beams 81 go down and return, the work X is positioned on the stationary beams. Thus, the intermittent sending is carried out on the work X. The lower part of the work X contacts either of the stationary beams 81 or the movable beams during operation, the contacting parts of the work X are not exposed to the atmosphere in the furnace at which unsatisfied parts would occur.
For lessening such disadvantages as possible, it may be considered that two stationary beams 80 are, as shown in FIG. 5(b), positioned in parallel according to the width of the work X, and the work X is supported on the beams 80 at the both sides of the under surface, and movable beams 81 are positioned adjacent to the stationary beams 80, so that the under surface of the work X widely contacts the atmosphere in the furnace.
However, if the work X is a base plate to be mounted thereon with tips of IC, LSI etc, it is difficult to keep excess parts in the work X by the amounts pf the contacting widths of the beams, since the width of the work X is small, and the tips are mounted in a full width of the work X.
The above mentioned is a big problem to not only the reflow furnace but also other atmospheric furnaces when the structures thereof are provided such that the both sides of the work contact the atmosphere therein for heightening reaction efficiency between the work and the atmosphere.